[TowerTalk] Dipole Secrets Finally Revealed
Roger L. Elowitz
Wed, 29 Apr 1998 01:36:08 -0400
We've been following this TowerTalk thread about the low inverted "V"
dipoles and it seemed to be an excellent jumping off point for further
field research, not unlike the kind we usually conduct here at the Elvis
Presley Memorial Antenna Radiation Institute for Unusual Propagation
Phenomena. You esteemed ladies and gentlemen on the TowerTalk reflector
have been searching for "THE SECRET" for improving the performance (ie.
gain and bandwidth) of ordinary dipoles and their variants, the Inverted
"V", and we felt that we could contribute our expertise to uncovering
some little known secrets.
First of all, we took the tact that everyone was possibly headed in the
wrong direction... as well intended people are often capable of doing.
For example, it has always been assumed that higher is generally better
for antenna performance. This was usually proved true up to a point...
and the literature is full of interesting diagrams relating the height
of the dipole above, not only physical ground... but also, above a
virtual image somewhere below ground. This in-the-ground image was
responsible for radiating an in-phase or out-of-phase signal with the
above ground wire creating interesting take-off angles that are
responsible for a good, great or a very poorly working antenna.
Therefore, it was the focus of our investigation to explore the true
nature of this in-ground reflection and to try to use it to our
advantage... revealing the SECRET OF THE DIPOLE! Compounding the
difficulty of our research was the fact that no one had ever seen the
in-ground image. Then again... no one had ever seen the above ground
image either. Our researchers tried everything including dark glasses,
polarized glasses, yellow lenses, RayBan lenses, and of course the
proverbial Coke bottle lenses. Nothing worked... yet the literature is
full of diagrams clearly showing reflections from below-ground images.
Our assumption for the sake of experimentation was that... the below
ground antenna image could NOT in fact reside far below ground due to
tree roots and water tables. We further theorized that if we took the
opposite tact of most antenna builders... and built our dipole below
ground... we could somehow get closer to the source of radiation from
the image antennas and therefore improve on the coupling and therefore
increase the dipole's efficiency. Not only that, but at the same time we
could immediately solve the problem of "unsightly" wires running across
our yards causing all manner of unwanted attention.
The problem for out test purposes was to be able to get the dipole
mounted at about a half wave below the ground in the approximate
location that the image was located. By co-locating the actual dipole
and the virtual image we theorized a 300% improvement in signal
performance at all take-off angles. This works out to a signal
improvement of 39-dbd. Since our wire dipole would be approaching the
actual location of a virtual radiation source we decided to call our
test assumption... our "null hypothesis."
Verifying this hypothesis is more a problem than can be imagined.
Getting the dipole a half wave below ground for an 80-meter dipole is no
small feat- far more difficult than getting it a half wave above ground.
Relay teams of moles had to be trained to dig straight down one hundred
and twenty-six feet at both ends and then straight across and meet each
other in the center. Then, once in the center they had to dig straight
up in order to provide an access for the feedline. The details of the
moles' training exceed the bounds of discussion on this reflector,
however, it could be mentioned in passing that it goes against the grain
for moles to burrow in this manner unless they are rewarded with Tootsie
Rolls to which they become readily addicted.
Finally, we had a team of draught moles pull the ends of the wire dipole
antenna down the center hole and when they reached the bottom, diverge
through the prepared tunnels towards the ends. As they did this the
center insulator with choke balun attached along with RG/U direct-bury
coax was pulled down the hole.
Finally we were all set to fire up the antenna after retrieving our mole
helpers. It was indeed necessary to retrieve our mole helpers since we
learned that treating moles unkindly by melting them with high levels of
RF exposure could be dangerous to us from a litigation point of view.
The Sierra Club and the Friends of the Earth Society take a dim view of
treating wildlife this cavalierly even if they were amply compensated
with Tootsie Rolls.
The big day arrived and we were standing by with field strength meters
in all directions at least ten wavelengths away. Low power initial
measurements indicated that the SWR was indeed 1:1 across the entire
75/80 meter band. Dead Flat!!! The only explanation for this that made
any sense at all was Steve's (K7LXC's) theory that with the feedline
going vertically down... caused all the SWR's to drained toward the
antenna and sucked any mismatch right out of the transmitter. It was,
as we later confirmed, a pure RF siphon effect.
As for the bandwidth, we attributed it to the compression of the
overlying earth squeezing the escaping RF into the available bandwidth
and not letting any escape as would normally happen in air or deep
space. Also, the thermal heating of the moisture in the soil
surrounding the antenna steamed the radiation pattern into a neat
take-off angle that could be varied by adjusting the temperature of the
superheated steam with the addition of some judicious above ground lawn
watering. By the way... the steamed earthworms we cooked tasted just
Talk about a killer stealth antenna!!!
This buried dipole antenna worked out like gangbusters. Signal reports
were off the scale... regardless of any power level we cared to use.
But alas, to our dismay, the project was a dismal failure. For
reception we heard absolutely NOTHING! Talk about quiet! All we could
hear was the internal thermal noise of our receiver. We had found the
ideal "one-way antenna" which is of use to absolutely no one.
And that ladies and gentlemen is the Secret of the Buried Dipole Antenna.
Respectfully submitted for your entertainment in the shadowy Twilight
Zone of TowerTalk.
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